Question

A greenish coating that develops on copper utensils is due to the formation of

• A. $\mathrm{CuO}$
• B. ${\mathrm{CuCO}}_3$
• C. $\mathrm{Cu}{\left(\mathrm{OH}\right)}_2$
• D. $\mathrm{Cu}{\left(\mathrm{OH}\right)}_2.{\mathrm{CuCO}}_3$

Answer 1

The correct option is D

$\mathrm{Cu}{\left(\mathrm{OH}\right)}_2.{\mathrm{CuCO}}_3$

The explanation for the correct option:

Option (D) :$\mathrm{Cu}{\left(\mathrm{OH}\right)}_2.{\mathrm{CuCO}}_3.$

• All copper vessels develop a green-coloured layer on their surface when exposed to moist air for a long time.
• In this chemical process, copper corrodes by oxidation, in which copper reacts with oxygen in the air to form copper oxide.
• Copper oxide then combines with carbon dioxide to make copper carbonate, which gives it a green colour.
• This process is called the corrosion of copper.
• The green material is a mixture of copper hydroxide ($\mathrm{Cu}{\left(\mathrm{OH}\right)}_2$) and copper carbonate $\left({\mathrm{CuCO}}_3\right).$
• The reaction given is:

$2\mathrm{Cu}\left(\mathrm{s}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{g}\right)+{\mathrm{CO}}_2+{\mathrm{O}}_2\to \mathrm{Cu}{\left(\mathrm{OH}\right)}_2+{\mathrm{CuCO}}_3\left(\mathrm{s}\right)$

The explanation for the incorrect option:

Option a) $\mathrm{CuO}$

• Copper oxide soon transforms into copper carbonate.
• So copper oxide doesn't exist as a product.
• Hence it is incorrect.

Option b) ${\mathrm{CuCO}}_3$

• Copper carbonate is not a product alone.
• It gives green colour along with calcium hydroxide.
• Thus it is incorrect.

Option c) $\mathrm{Cu}{\left(\mathrm{OH}\right)}_2$

• Copper hydroxide with calcium carbonates appears to be green in colour.
• This is incorrect.

Hence, a greenish coating that develops on copper utensils is due to the formation of $\mathrm{Cu}{\left(\mathrm{OH}\right)}_2.{\mathrm{CuCO}}_3.$